‘Humongous fungus’ is almost as big as the Mall of America

In the late 1980s, researchers discovered the biggest organism on record, a “humongous fungus” on Michigan’s Upper Peninsula that covered 37 hectares, about the same size as the Mall of America in Bloomington, Minnesota. Now, the same team of scientists has found that this Armillaria gallica, which gives rise to honey mushrooms (above), is about four times as big—and twice as old—as they originally thought.

Like other fungi, Armillaria sprouts tiny threads underground; but unlike most fungi, these threads fuse to form shoelace-size strings that extend great distances to consume dead or weak wood. To find out how big the massive underground network of fungus really was, the scientists took 245 far-flung string samples and analyzed their genes. Not only did they belong to the same individual fungus, but—based on how fast the underground strands grow—that fungus must be at least 2500 years old, they report in a non–peer-reviewed study posted last week to the bioRxiv preprint server.

By sequencing the genomes of 15 evenly distributed samples, the researchers could also see how the honey mushroom’s genome changed over time. To their surprise, it has a very slow mutation rate, with just 163 genetic changes among the genome’s 100 million bases. Mutation rates often reflect how quickly an organism can evolve—and this fungus, it seems, doesn’t evolve very fast. The researchers aren’t sure how the mutations rate is kept in check, though a well-developed DNA mechanism or simply being underground and out of sunlight may do the trick.

But even with its new size estimate, the Michigan fungus has already been eclipsed by a different Armillaria in Oregon, discovered in 1998. That one, now the largest organism in the world, may be more than 8000 years old and covers more than 770 hectares.